The present invention pertains to systems for detecting backscattered electrons propagating from an object being irradiated with an electron beam or other charged particle beam. Such detection systems are utilized, for example, in charged-particle-beam microlithography apparatus and methods requiring accurate alignment of a reticle pattern with a desired projection location on a substrate. More specifically, the invention pertains to backscattered-electron detection systems that produce a high signal-to-noise (S/N) ratio for greater detection (and hence alignment) accuracy.
A conventional backscattered-electron (BSE) detector system is shown in FIGS. 9(a)-9(b). Such BSE detector systems are typically included with electron-beam microlithography apparatus for use in, e.g., aligning the silicon substrate prior to the substrate being exposed by electron-beam irradiation.
In FIG. 9(a), a sample 107 comprises a substrate 104 made of a relatively xe2x80x9clightxe2x80x9d (low mass) element such as silicon. Pattern features 105 are formed on the surface of the sample 107. The pattern features 105 are made of a relatively xe2x80x9cheavyxe2x80x9d (high mass) element such as tantalum. An electron beam EB incident to the surface of the substrate 107 is scanned in the direction of the xe2x80x9cscanxe2x80x9d arrow along the dashed line 108. As the incident electrons in the beam EB penetrate into the sample 107, they experience many scattering events that produce backscattered electrons 103. The backscattered electrons 103 are detected by BSE detectors 101 and 101xe2x80x2.
The signal produced by the BSE detectors 101, 101xe2x80x2 as the electron beam EB passes over the feature elements 105 is profiled in FIG. 9(b). FIG. 9(b) also depicts a representative relationship between the specific location on the sample 107 being irradiated by the electron beam EB versus the amplitude of the electrical signal produced by the BSE detectors 101, 101xe2x80x2.
Conventional BSE detectors 101, 101xe2x80x2 commonly comprise PN-junction or PIN-junction semiconductors for detecting backscattered electrons. The detector junction is biased and, as backscattered electrons impinge on the detector junction, corresponding changes in current or voltage flowing through the junction are produced to generate a detector signal. The amplitude of the detector signal is a function of the energy of the corresponding incident backscattered electrons. The detector signal is normally amplified.
The energy of electrons backscattered from the sample 107 differs according to the specific material on which the electron beam EB is incident (e.g., on a pattern feature 105 versus on the surface of the substrate 104). Hence, as the electron beam EB encounters different materials on the sample 107, the amplitude of the signal produced by the BSE detectors 101, 101xe2x80x2 changes. This phenomenon is exploited for detecting, using the BSE detectors 101, 101xe2x80x2, the presence and position of a pattern on the sample 107. For example, prior to performing a microlithographic exposure of the substrate 104 using the electron beam, the location of an alignment mark on the substrate 104 is detected, using the BSE detection system of FIGS. 9(a)-9(b), so as to positionally align the substrate for exposure.
A general object of the invention is to increase the accuracy with which a pattern feature on a sample can be detected, compared to the performance of the conventional BSE detector system summarized above. The initial approach was to increase the differences in energy exhibited by backscattered electrons from various regions on the sample. As noted above, the energy of electrons backscattered from a substance is characteristic of the substance and is normally different for different substances. Another variable that influences the energy of backscattered electrons is the thickness of the substance on which the beam is incident. Hence, in order to increase the difference in energy of electrons backscattered from various materials, increasing the thickness of pattern features (e.g., elements of an alignment mark) was initially considered.
However, modern methods for fabricating semiconductor devices typically include one or more planarization steps, generally performed by chemical-mechanical polishing or an analogous technique. This has made it difficult to provide a suitably thick alignment-mark pattern, for example, for obtaining a desired large difference in energy of backscattered electrons as the electron beam passes over an alignment mark on a planarized surface.
In view of the above, an object of the present invention is to provide BSE detection systems capable of producing and detecting large changes in detection-signal amplitude as the beam is incident on various materials on the sample surface. The resulting detection signals produced by such systems have an increased signal-to-noise ratio and increased position-detection accuracy.
According to a first aspect of the invention, detection systems are provided for detecting electrons backscattered from a locus on a sample surface irradiated with an electron beam. A representative embodiment of such a system comprises first and second BSE detectors. The first BSE detector has a first prescribed xe2x80x9cenergy-sensitivity bandxe2x80x9d (i.e., the first BSE detector is sensitive to electrons having respective energies within a first energy range), and is configured and situated so as to receive a first group of backscattered electrons propagating from the locus due to irradiation of the locus with the electron beam. The first BSE detector produces, from the received backscattered electrons, a first electrical signal. Similarly, the second BSE detector has a second prescribed energy-sensitivity band (i.e., the second BSE detector is sensitive to electrons having respective energies within a second energy range) that is different from the first energy-sensitivity band. The second BSE detector is configured and situated so as to receive a second group of backscattered electrons propagating from the locus due to irradiation of the locus with the electron beam, and to produce from the received backscattered electrons a second electrical signal. The system includes a signal combiner connected to the first and second BSE detectors. The signal combiner is configured to combine the first and second signals together yielding a composite output-signal waveform having an amplitude that differs depending upon a characteristic of the locus.
A typical characteristic of the locus is the specific material of the locus. For example, the sample can comprise a substrate surface including a pattern feature. In such an instance, the output signal produced by the signal combiner has a waveform corresponding to irradiation of the substrate surface and the pattern feature with the electron beam.
According to a first example embodiment of a detection system according to the invention, the first and second BSE detectors are solid-state detectors, in which each first and second BSE detector comprises a PN or PIN junction with a respective depletion layer. Desirably, at least one of a depth and width of the respective depletion layer is adjustable so as to adjust the energy-sensitivity band of each BSE detector. If the output signal waveform of the first BSE detector is denoted SA and the output signal waveform of the second BSE detector is denoted SB, then the signal combiner produces a composite signal according to the expression: SAxe2x88x92xcex1SB.
Multiple first and second solid-state BSE detectors can be used. For example two first BSE detectors can be used, designated xe2x80x9cAxe2x80x9d and xe2x80x9cAxe2x80x2xe2x80x9d, and two second BSE detectors can be used, designated xe2x80x9cBxe2x80x9d and xe2x80x9cBxe2x80x2xe2x80x9d. In such an instance, the combiner produces a composite output signal waveform according to the expression:
(SA+SAxe2x80x2)xe2x88x92xcex1(SB+SBxe2x80x2)
wherein SA and SAxe2x80x2 denote respective output waveforms from the first solid-state detectors A, Axe2x80x2, and SB and SBxe2x80x2 denote respective output waveforms from the second solid-state detectors B, Bxe2x80x2, and xcex1 denotes a coefficient.
The number of detectors is not limited to one or two for each energy-sensitivity band. By way of example, four detectors for each energy-sensitivity band are possible and work well. In general, xe2x80x9cnxe2x80x9d detectors can be provided for the first energy-sensitivity band and xe2x80x9cmxe2x80x9d detectors can be provided for the second energy-sensitivity band, wherein n and m are integers that desirably, but not necessarily, are equal. Under such conditions, the expression above is of the form:
(SA1+SA2+ . . . +SAn)xe2x88x92xcex1(SB1+SB2+ . . . +SBm)
wherein SAi (i=1, 2, . . . , n) denotes an output signal waveform from a respective first detector and SBi (i=1, 2, . . . , m) denotes an output signal waveform from a respective second detector, and xcex1 denotes a coefficient.
According to a second example embodiment of a detector system according to the invention, each first BSE detector is a solid-state detector comprising a detection surface, and each second BSE detector comprises an electrically conductive film situated upstream of the detection surface of the solid-state detector. The film is configured to cut off certain backscattered electrons from the surface of the sample. The film desirably has a thickness suitable for absorbing or otherwise cutting off backscattered electrons having an energy less than at least a peak of an energy distribution of the backscattered electrons produced by the irradiated sample surface. For example, if the sample surface is irradiated with an electron beam accelerated by a potential difference of no greater than 100 KV, the film can be made of silicon having a thickness of at least 10 xcexcm but not greater than 40 xcexcm. As an alternative to silicon, the film can be made of metal.
The backscattered electrons from the surface of the sample referred to above desirably arise from portions of the sample surface other than the irradiated pattern feature. It is desirable that the thickness of the thin film be such that, of the backscattered electrons produced by the surface of the sample, backscattered-electrons having the highest energies (or nearly the highest energies) are absorbed or otherwise cut off by the thin film.
According to a third example embodiment of a detector system according to the invention, the first BSE detector is a solid-state detector comprising a detection surface, and the second BSE detector comprises an electrically conductive mesh situated upstream of the detection surface of the solid-state detector. The mesh desirably is charged with a positive voltage so as to decelerate backscattered electrons passing through the mesh. Further desirably, the positive voltage is sufficient to cut off an energy band of backscattered electrons from the surface of the sample. For example, the positive voltage can be sufficient to cut off backscattered electrons having an energy less than at least a peak of an energy distribution of the backscattered electrons produced by the irradiated sample surface. Desirably, the positive voltage corresponds to the approximately highest energy level within the energy distribution of backscattered electrons produced by the sample surface. By way of example, the sample is irradiated with an electron beam accelerated by a potential difference of no greater than 100 KV. In such an instance, the positive voltage is at least 40 KV and not greater than 70 KV.
According to another aspect of the invention, detection systems are provided for detecting electrons backscattered from a sample surface irradiated with an electron beam, wherein the sample surface includes a pattern feature. Such a system comprises at least two types of backscattered-electron detectors each having a mutually different energy-sensitivity band. The system also comprises a means for selecting for use, among the two or more types of BSE detectors, the one or two types of BSE detectors best suited to the characteristics of the sample.
According to another aspect of the invention, methods are provided for detecting electrons backscattered from a locus on a sample surface irradiated with an electron beam. A representative embodiment of such a method comprises, as a first step, detecting from the locus backscattered electrons having respective energies within a first energy band and producing a corresponding first electrical signal. Backscattered electrons from the locus are also detected having respective energies within a second energy band, from which a corresponding second electrical signal is produced. The first and second electrical signals are combined to produce a composite output signal waveform having an amplitude that differs depending upon a characteristic of the locus.
According to another embodiment of a method according to the invention, a first BSE detector is provided having a first prescribed energy-sensitivity band, and a second BSE detector is provided having a second prescribed energy-sensitivity band different from the first energy-sensitivity band. The first BSE detector is positioned so as to receive a first group of backscattered electrons propagating from the locus due to irradiation of the locus with the electron beam. A first electrical signal is produced corresponding to the energy-sensitivity band of backscattered electrons detected by the first BSE detector. The second BSE detector is positioned so as to receive a second group of backscattered electrons propagating from the locus due to irradiation of the locus with the electron beam. A second electrical signal is produced corresponding to the energy-sensitivity band of backscattered electrons detected by the second BSE detector. The first and second electrical signals are combined to produce a composite output signal waveform having an amplitude that differs depending upon a characteristic of the locus. By combining the first and second electrical signals, a desired detection signal waveform can be obtained exhibiting large signal-level changes as the electron beam is scanned across the sample.
The foregoing and additional features and advantages of the invention will be more apparent from the following detailed description, which proceeds with reference to the accompanying drawings.